1. Field of the Invention
This invention relates to a method for determining the feed water measurement in a steam generator such as those used in pressurized water reactors and, more particularly, to a method for using a differential-type transmitter to sense the feed water level in a steam generator in a nuclear power plant.
2. Related Art
A typical nuclear steam generator comprises a vertically-oriented shell; a plurality of U-shaped tubes disposed in the shell so as to form a tube bundle; a tube sheet for supporting the tubes at the ends opposite the U-like curvature; a dividing plate that cooperates with the tube sheet forming a primary fluid inlet header at one end of the tube bundle and a primary fluid outlet header at the other end of the tube bundle; a primary fluid inlet nozzle in fluid communication with the primary fluid inlet header; and a primary fluid outlet nozzle in fluid communication with the primary fluid outlet header. The steam generator also comprises a wrapper disposed between the tube bundle and the shell to form an annular chamber adjacent the shell, and a feed water ring disposed above the U-line curvature end of the tube bundle. The primary fluid, having been heated by circulation through the nuclear reactor core, enters the steam generator through the primary fluid inlet nozzle. From the primary fluid inlet nozzle, the primary fluid is conducted through the primary fluid inlet header, through the U-tube bundle, out the primary fluid outlet header and through the primary fluid outlet nozzle to the cold leg of the reactor coolant loop. At the same time, feed water is introduced to the steam generator through the feed water ring. The feed water is conducted down the annular chamber adjacent the shell until the tube sheet near the bottom of the annular chamber causes the feed water to reverse direction, passing in heat transfer relationship with the outside of the U-tubes and up through the inside of the wrapper. While the feed water is circulating in heat exchange relationship with the tube bundle, heat is transferred from the primary fluid in the tubes to the feed water surrounding the tubes causing a portion of the feed water to be converted to steam. The steam then rises and is circulated through a series array of moisture separators, which dries out the steam before the steam is conveyed to steam turbine and condensation equipment, which are respectfully employed to drive a generator to produce electricity and to return the condensed fluid back to the steam generator feed water inlet.
The current steam generator feed water level measurement system is more fully described in U.S. Pat. No. 5,024,802. The current steam generator feed water level measurement methodology relies on the use of a differential-type transmitter for comparing the steam generator feed water level with a reference leg pressure input. The transmitter responds to a differential of water pressures inputted to it and provides an output representative of the difference between the reference leg pressure and the pressure due to the height of the liquid in the steam generator. This differential is a good correlation to water level when the lower pressure tap, for measuring the water level in the steam generator, is located in a relatively low velocity region of the steam generator, e.g., above the transition section of the steam generator shell. However, if the lower pressure tap is moved to a higher velocity region, e.g., below the transition portion of the shell, an error is introduced due to the effective velocity head of the moving water. In such circumstances, a level measurement penalty must be taken, meaning that the steam generator operating level margin, i.e., a range of permissible water levels, needs to be reduced. However, it is preferable to locate the lower tap in a high velocity region so as to minimize what is known as the shrink/swell phenomenon.
The control of feed water level in the secondary loop of a nuclear steam supply system is exemplary of situations where control is difficult because the system behaves with non-minimum phase dynamics. Non-minimum dynamics is a term used to describe a property of the frequency domain transfer function between plant input function and plant output function. Transport lags or pure-time delay between an input signal and its corresponding output is one form of non-minimum phase behavior. Another form of non-minimum phase behavior is an initial negative response of an output signal before changing sign and approaching its positive asymptote. This type of non-minimum phase behavior is what is often called by operators of such nuclear steam supply systems “shrink/swell behavior”.
Changes in reactor power, steam flow, feed water temperature and feed water flow all affect the measured level of secondary loop feed water contained in the steam generator. The feed water level controller's basic task is, therefore, to maintain the secondary side steam generator water level on target and within limits by changing feed water flow to compensate for changes in level produced by the other factors. The main consequence of the long lags and shrink and swell effects is that a controller must anticipate the affects of changes in plant state or control actions on steam generator level, and make compensatory responses before the ultimate affect of the event on steam generator level is manifested in measured level. Accordingly, it is very desirable to minimize the shrink/swell phenomenon by locating the lower pressure tap in a region such that the velocity head will be about 6 in. to 8 in.
U.S. Pat. No. 5,024,802 teaches a method for compensating for the velocity head effects experienced by the lower tap in high velocity regions by adding compensation during calibration so as to reduce the margin added to the high level trip set point to account for uncertainty in water level. This invention recognizes the need to further compensate the differential-type pressure transmitter signal for the pressure-altering effects of the feed water level caused by certain structural restrictions that impact the feed water pressure experienced at the lower tap, such as the middle deck plate pressure delta.